Which of these is commonly used as a protective layer before nail polish?

A base coat is commonly used as a protective layer before applying nail polish. Its primary function is to create a smooth surface for the polish to adhere to, enhancing the overall finish and longevity of the manicure. The base coat also helps to prevent staining of the natural nail from colored polishes and can fill in any ridges or imperfections, ensuring an even application of the color.

In contrast, a top coat is applied after the nail polish to seal in the color and provide a glossy finish, along with added protection against chipping and wear. Cuticle oil is utilized to nourish and moisturize the skin around the nails, promoting healthy cuticles but does not serve as a protective barrier for polish application. Nail hardener is designed to strengthen weak nails but is not specifically formulated to act as a base for nail polish. Each of these products serves different purposes in nail care and aesthetic enhancement, emphasizing the importance of the base coat as the foundation for a polished look.